Chemostratigraphic correlation and Pb-Pb dating of carbonate sequences in the external Gariep belt and Kango inlier of the Saldania belt in Namibia and South Africa

Abstract

Abstract by author:

Application of a refined 207Pb-204Pb double-spiking technique to correct for mass-fractionation led to marked improvement in the quality of Pb isotope data and resulting ages obtained on low-grade metamorphosed carbonate rocks from the Port Nolloth and Kango Groups. Carbonate rock samples from the lower Pickelhaube Formation of the Hilda Subgroup, Port Nolloth Group, yielded distinctly different 207Pb/206Pb isochron ages of 728 ± 32 Ma (MSWD = 1. 5) and 545 ± 13 Ma (MSWD = 7. 4) for the carbonate and residue fractions, respectively. The former is interpreted as dating early diagenesis, whereas the latter is in excellent agreement with the timing of regional metamorphism. In contrast, both the carbonate and residue fractions from the Bloeddrif Member, Holgat Formation, Port Nolloth Group, and from the Kombuis Member, Matjies River Formation, Kango Group, yielded indistinguishable 207Pb/206Pb isochron ages. The isotope data from both fractions combined define an isochron age for the Bloeddrif Member limestone of 555 ± 28 (MSWD = 22. 8). A whole rock isochron age of 553 ± 30 Ma (MSWD = 14. 3) is given by the weighed sums of separately analysed carbonate and residue fractions data of the Kombuis Member. Both ages are interpreted to reflect early diagenesis. The novel approach to double-spike 207Pb/206Pb dating of carbonate rocks highlights the importance of dating carbonate and residue fractions separately to obtain geologically meaningful age data

Carbonate rock samples from all important carbonate bearing sequences of the Port Nolloth and Kango Groups were analysed for d13CCarb (carbonate C), 87Sr/86Sri (initial Sr) and for their major and trace elemental compositions. Highly positive d13Ccarb anomalies (up to 8. 65 0/00) together with consistently low 87Sr/86Sri ratios (0. 7071 - 0. 7076) recorded in carbonate successions of the Hilda Subgroup (Port Nolloth Group) between the older Kaigas and younger Numees Formation diamictites are in agreement with data recorded for carbonate sequences elsewhere that were deposited between the global Sturtian and Varangian glaciations (~720 - 600 Ma). This is supported by the 728 ± 32 Ma Pb-Pb age for the lower Pickelhaube Formation carbonate rocks. Considerably lower, positive d13CCarb values (up to 1. 01 0/00) and higher 87Sr/86Sri ratios (0. 7082 - 0. 7085) and the 555 ± 28 Ma Pb-Pb age for the Bloeddrif Member (Holgat Formation) on top of the Numees Formation diamictite yield strong evidence that the Holgat Formation is post-Varangian in age. Correlation of the Kaigas and Numees Formation diamictites with the global Sturtian and Varangian glaciations, respectively, as indicated by this study implies unusually low sedimentation rates of ~ 1. 5 - 3 m/myr for the carbonate sequences of the Hilda Subgroup

On the basis of d13Ccarb and 87Sr/86Sri data obtained on other Pan-African successions in the region (Gifberg Group, Kango Group), chemostratigraphic correlation between these groups and the Port Nolloth Group became possible. The Widouw Formation limestone (Gifberg Group) is correlated with the Bloeddrif Member. At the bottom of the Kango Group, the carbonate bearing Nooitgedacht Member (lower Matjies River Formation) compares well with the Hilda Subgroup. In contrast, the overlying Kombuis Member (upper Matjies River Formation) shows strong similarities to the Bloeddrif Member. Such a correlation is supported by almost identical Pb-Pb ages of 553 ± 30 Ma and 555 ± 28 Ma obtained on the Kombuis and Bloeddrif Member carbonates

Correlation of the Nooitgedacht Member with the pre-orogenic Hilda Subgroup leads to the recognition that the former represents the only pre-orogenic Pan-African succession in the whole of the Kango Group. The Kombuis Member and Holgat Formation are syn-to post-orogenic with respect to the Pan-African orogeny, showing that the existing stratigraphic scheme of the Kango Group is in need of fundamental revision

Two regional metamorphic events that affected the rocks in the Kango inlier are distinguished based on calcite-graphite C isotope thermometry on carbonate rock samples from the Nooitgedacht and Kombuis Members. Temperatures of 380 - 398 °C obtained for carbonate rocks from the Nooitgedacht Member most likely reflect the peak of Pan-African metamorphism. Considerably lower temperatures of 270 - 299 °C for the Kombuis Member carbonate rocks are interpreted to be related to the Cape orogeny, further emphasising that the bulk of the Kango Group is post-orogenic with respect to the Gariepian/Saldanian orogeny